The quality of an extrusion of an aluminum billet is dependent upon, among other things, the temperature of the billet during the extrusion operation and the speed with which the extrusion operation proceeds. Generally, it is known that as the temperature of the billet immediately prior to extrusion increases, the speed with which the extrusion process may proceed decreases. However, the lower temperature may also result in defects in the extruded product as a result of the hard and brittle MgSi phases within the aluminum matrix. Obviously, it is most desirable to maximize the extrusion speed to increase the productivity of the extrusion apparatus. However, the extrusion speed is limited by the temperature of the billet and the existence of hard incipient phases in the matrix. As described in an article by Oddvin Reiso entitled, "The Effect of Billet Preheating Practice on Extrudability of AlMgSi Alloys," published in the 1988 Proceedings of the Fourth Intermediate Aluminum Extrusion Technology Seminar, defects in the extruded billet can be minimized by heating an AlMgSi billet above the solutionizing temperature to dissolve any MgSi phases contained within the aluminum matrix prior to extrusion. Thereafter, the billet is cooled somewhat to an adequate working temperature, but not sufficient to allow for significant reformation of the solutionized MgSi phases. At this working temperature, the billet may be extruded at a maximum extrusion speed with minimal defects in the extruded product.
It is also well known to create a temperature differential throughout a metal billet prior to an extrusion operation in order to eliminate defects in the extruded products and to create more uniform properties throughout the extruded product. The temperature gradient is created between the ends of the billet. The temperature gradient may be used to create an extrusion which has a uniform temperature upon exiting the extrusion die by compensating for the heat created in the billet as a result of the extrusion operation. The temperature gradient may also be utilized to help remove impurities such as air from the billet during the extrusion operation through selective deformation of the billet.
The U.S. Pat. No. 2,639,810 to Doan (issued May 26, 1953) discloses the concept of extruding a metal billet having a temperature gradient between the ends. The metal billets are pretreated to establish a temperature gradient extending from a hot working temperature at one end to a materially lower temperature at the other end While this gradient exists, the billet is extruded with the hot end adjacent the die with the result that any air in the cylinder is expelled positively from around the billet rearwardly along the ram and out of the press. The temperature gradient can be created by spraying one end of the billet with water or by standing the billet on one end momentarily in a shallow pool of water after heating the billet to a uniform hot-working temperature.
A further example of creating a temperature gradient throughout a heated workpiece is disclosed in the U.S. Pat. No. 2,409,422 to Egan (issued Oct. 15, 1946). The patent discloses a means for creating a temperature differential in a bi-metallic billet in a hot-rolling operation by subjecting one of the components of the billet to spray from a cooling medium.
Use of a temperature gradient is also disclosed in U.S. Pat. No. 2,480,774 to Rossheim, et al. (issued Aug. 30, 1949) for application in bending of thin walled thermoplastic bodies including tubes. The gradient is created by employing heating means that circumscribe the tube and cooling rings on either side of the heating means. The U.S. Pat. No. 3,902,334 to Stuart (issued Sept. 2, 1975) also discloses the use of heating and cooling rings which circumscribe a tube to allow for uniform bending of the tube.
An extrusion operation which measures the temperature of the extruded product and utilizes sprays from various cooling mediums to cool the extruded product is disclosed in U.S. Pat. No. 2,863,557 to Munker (issued Dec. 9, 1958). The temperature sensing means are used to control the speed of the extrusion. This concept is also found in U.S. Pat. No. 3,212,309 to Wilson (issued Oct. 19, 1965). In the Wilson patent, a temperature sensing device is used to control the speed of a wire drawing machine and the amount of coolant applied to the drawn wire.
The U.S. Pat. No. 2,964,834 to Schober (issued Dec. 20, 1960) also utilizes a temperature gradient within a metal blank to create desired properties in the end product in a forging operation. This patent discloses creating a temperature gradient within a metal blank prior to a forging operation to allow for free and uniform movement of the metal to the extreme portions of the forging die as the force of the press is applied. This method also creates a one-directional grain structure at the extremities of the die.
The concept of applying coolant to an extruded article immediately after the extrusion operation is found in U.S. Pat. No. 3,739,619 to Follrath, et al. (issued June 19, 1973).